This invention relates to a coupling structure for connection between waveguides or between a waveguide and an optical fiber formed on respective substrates, and relates to a process of forming the coupling structure.
It is easy for optical waveguides to integrate the optical branching and linking functions and the optical synthesis and dissolution functions, and to link with optical semiconductor elements. In addition, downsizing and lower price, because of their advantage of being superior in mass-production, can be expected. To apply optical waveguides to devices for the optical-fiber communication, connection between optical waveguides and optical fibers is essential. Also for making effective use of the characteristic of the optical waveguides, a simple, high-reliable coupling structure for connection between an optical waveguide and an optical fiber is needed. Furthermore, to build a large-scale integrated circuits, there can occur some cases where waveguides formed on respective substrates must be interconnected.
One of the most simplified coupling structures for connection between an optical waveguide and an optical fiber is such that an optical fiber is firmly adhered, as with an adhesive, in the optical-axis adjusted state to an optical waveguide substrate with the ends worked as mirror-surface. Another structure shown in FIG. 1 is in Japanese Laid-Open Patent No. Hei.3-50510 in which optical-fibers with one ends thereof worked into a lens-shape are located at a fixed distance from the one-ends of the optical guides and position-adjustment is made between them. For such connection structure it is needed to make the optical-axis alignment while monitoring the optical output so that the core of the optical fiber may be aligned with the core of the optical waveguide. It therefore has defects such as very poor productivity, misalignment due to change in ambient temperature and degradation with time as the result of merely bringing the optical fiber into face to face contact with the optical waveguide substrate and adhering with an adhesive, and the consequent low reliability.
A technique for removing the later-described defect is known, for example, from an article (Reference 1): C-186 of the Autumn (1992) Meeting proceedings of the Electronics/Information/Communication Institute, which consists of carrying out the optical-axis alignment of an optical fiber with an optical waveguide, followed by irradiation of of CO.sub.2 laser at the ends of them contacted with each other to be fused together.
The technique described in reference 1 however is encountered in problems of requiring optical-axis alignment of the optical fiber, being essential for the optical waveguide substrate to be made from fusible glass, and resulting in a poor strength because of being the restricted area to be fused together and thus an insufficient reliability. It has another drawback of being impossible to be applied to connection between optical waveguides though application to connection between an optical waveguide and an optical fiber is possible.
To solve the defects of these conventional connection techniques that need the positional adjustment, a further structure was proposed which permits coupling between an optical fiber and an optical waveguide without need for the positional adjustment by means of forming a V-shaped groove on the substrate of the optical waveguide and placing the optical fiber on this V-shaped groove as shown in FIG. 2.
This structure however has a defect that does not allow connection between optical waveguides without adjustment. Notably for forming a large-scale optical integrated circuit, two or more connections between optical waveguides are needed, and this has been very difficult so far.